Background
Condensed tannins are naturally-occurring secondary carbon compounds produced in the leaves of a number of different plants that sometimes act to deter herbivorous insects. In addition, scientists with New Zealand's AgResearch Grasslands institute have found that sheep and cattle feeding on forage containing condensed tannins may emit less methane, which is the second most important atmospheric greenhouse gas behind carbon dioxide, thereby providing another beneficial environmental impact.

What was done
Two field-growing silver birch (Betula pendula Roth) clones (4 and 80) were exposed, via open-top chamber, to either ambient-CO2 (350 ppm) air or elevated-CO2 (700 ppm) air for three growing seasons, over which period (among other things) a number of phenolic compounds of naturally-abscised leaf litter were identified and quantified, among which were condensed tannins.

What was learned
In terms of weight per unit mass of naturally-abscised silver birch leaves, it was found that leaf concentrations of total phenolics in clone 4 were 27% greater in the elevated-CO2 treatment compared to the ambient-CO2 treatment, while leaf concentrations of condensed tannins were also found to be 27% greater in the elevated-CO2 treatment, while in clone 80, the CO2-induced leaf concentration increases were determined to be 19% for total phenolics and 18% for condensed tannins.

What it means
Kasurinen et al.'s findings provide yet another example of atmospheric CO2 enrichment possessing the potential to (1) help reduce deleterious insect plagues of forests, and (2) help slow the rate of rise of the atmosphere's methane concentration, which over the last few years has amazingly come to a screeching halt (see Methane (Atmospheric Concentrations) in our Subject Index).